Project Summary Heart failure occurs when the cardiac muscle is weakened and cannot pump sufficiently to meet the body's need for blood and oxygen. Heart failure affects approximately 6 million of Americans and becomes a tremendous burden on our healthcare and economy system. Hypertension is one of the most prominent risk factors of heart failure. In response to high blood pressure, ventricular wall stress is augmented to overcome the increase of afterload pressure. The heart then manifests parallel growth to ameliorate wall stress. This concentric hypertrophic growth, once adaptive, may lead to fibrosis, inflammation, cardiac dysfunction and eventually heart failure. Despite the important of this devastating disease, our understanding is incomplete. Multiple events in heart failure progression are potent inducers of the unfolded protein response (UPR), a cellular adaptive process to cope with protein-folding stress. Three signaling transducers participate in the UPR to increase protein-folding capacity, reduce load of protein-folding and degrade terminally misfolded proteins. However, the role of the UPR in pressure overload-induced cardiac hypertrophy and heart failure remains to be defined. Preliminary work shows that Xbp1s, the most conserved branch of the UPR from yeast to mammals, is acutely and potently induced in heart. Overexpression of Xbp1s in cardiomyocyte is sufficient to cause hypertrophy. GFAT1, the rate-limiting enzyme of the hexosamine biosynthetic pathway, is discovered as a novel transcriptional target of Xbp1s. Inducible overexpression of GFAT1 leads to more profound response to pressure overload. GFAT1, and the hexosamine biosynthesis, may therefore mediate Xbp1s-induced hypertrophic growth. Moreover, Xbp1s overexpression leads to strong activation of mTORC1, an essential player in nutrient sensing and cell growth. Xbp1s may therefore couple the UPR, protein-folding, hexosamine biosynthesis and cell growth. Studies proposed here aim to define the role of the Xbp1/GFAT1/mTORC1 axis in cardiac hypertrophy and pathological remodelling in response to pressure overload. Both gain- and loss-of- function approaches using inducible systems will be employed in rodents. Comprehensive analysis for cardiac function, histological changes, and molecular derangements will be conducted. In vivo work will be corroborated by in vitro experiments with isolated neonatal myocytes to further decipher underlying mechanisms. Elucidation of the role of Xbp1s/GFAT1 in cardiac hypertrophy and pathological remodelling will greatly advance our understanding of the pathology of heart failure and pave a way for future clinical applications.